The present invention relates to a vacuum cartridge for an electrical protection apparatus such as a switch or a circuit breaker, said cartridge comprising an enclosure of substantially cylindrical shape closed by two end plates, two contacts extending axially inside the enclosure, at least one whereof, called the movable contact, is connected to an operating mechanism and is mounted sliding between a closed position of the contacts enabling the current to flow and a position wherein the contacts are separated and form an electric arc between them, and a means for producing an axial magnetic field in the arc formation zone. A vacuum circuit breaker of the above-mentioned kind is described in the document FR 2,682,808 or FR 2,726,396 filed by the applicant.
In this type of circuit breaker, the vacuum cartridges have under strong currents a breaking capacity which is limited by the following phenomenon. When the current flows, the arc moves liquid matter coming from the melting contact to the edge of the contacts. This very hot liquid matter (about 2000xc2x0) is situated at the edge of the contacts at the moment breaking occurs. After breaking, the recovery voltage imposed by the power system is established between the two contacts. On account of the geometric shape of the contacts, the electric field generated by this voltage is maximum at the edges of the contacts. This gives rise to the following undesirable phenomena. Firstly, when cooling, the liquid continuously gives off metallic vapors and gases. These emissions create an atmosphere with a high gas density which, in an environment normally in a strong vacuum, has a reduced dielectric strength. Secondly, the electric field, present just above the liquefied and very hot matter, gives rise to emission of electrons by a thermoelectric mechanism. These free electrons ionize the vapor and the gas. The combination of these two phenomena then leads to rearcings and to failure of the cartridge in breaking. In addition, the electric field is liable to deform the surface of the liquid increasing the electric field even further. This is a detrimental phenomenon that is self-amplifying and that can lead to rearcing.
To overcome these drawbacks in industrialized cartridges, certain measures have been taken that consist in respecting a distance, called the optimal distance, between the contacts and the wall of the enclosure, which distance guarantees a maximum breaking capacity. This therefore imposes constraints as to the volume of the cartridge.
Furthermore, some of the cartridges of the kind referred to above, as described in the document FR 2,726,396, in addition comprise a metal shield acting as the wall of the enclosure. This shield is connected to one of the contacts, more often than not to the stationary contact, and secondarily produces an increase of the electric field created at the level of the movable contact. Thus, dielectrically, the cartridge is considered as being asymmetric. The electric field is stronger at the edges of the movable contact than at the edges of the stationary contact. And, depending on the polarity of the recovery voltage, the dielectric strength after breaking is more or less good. Thus it is preferable for it to be the stationary contact that is subjected to a negative voltage rather than the movable contact. When the movable contact is subjected to a negative voltage with respect to the stationary contact, the hot matter emits more electrons due to the presence of the stronger electric field, which increases the risks of rearcing. This asymmetry in terms of geometry is also expressed by an asymmetry in terms of breaking capacity, the difference being about 15% for a rated current of 25 kA rms.
The present invention overcomes these problems and proposes a vacuum cartridge with improved breaking capacity and whose behavior on breaking is more symmetric. The cartridge achieved according to the invention can also be reduced in size.
For this purpose, the object of the present invention is to achieve a vacuum cartridge of the previously mentioned kind, this cartridge being characterized in that it comprises at least one conducting shield fitted around the periphery of at least one of the contacts, said shield being shaped and arranged in such a way as to withstand the electric field arising from the recovery voltage imposed by the power system after breaking and to thus considerably reduce the electric field at the edge of the contact.
According to a particular embodiment, the above-mentioned shield is electrically connected to said contact and is sufficiently separated from said contact for the liquid running on the contact surface of said contact not to be able to reach said shield. For this purpose, a trench separates the contact surface of the contact from the edge of said shield.
According to a particular feature, the above-mentioned shield is present around the whole of said contact.
According to anothter feature, the above-mentioned shield presents a semi-annular shape and is fixed onto the bottom part of the movable contact.
According to another feature, the cartridge comprises, for one or each contact, a shield called the first shield comprising the features mentioned above taken either alone or in combination, and a shield called the second shield, fitted between the shield called the first shield and the enclosure of the cartridge, said second shield being designed to protect the enclosure from metallization by the arc vapor and secondarily producing an increase of the value of the electric field on said contact.
According to a particular feature, the physical continuity is achieved on the contacts in the closed position at a place where the wear of the contacts is minimal.
According to a particular feature, the physical continuity is achieved by means of a ring, sectors of ring or studs arranged on the contact surface of one of the contacts and situated respectively facing a ring, sectors of ring or studs arranged on the contact surface of the other of the contacts.
According to a particular feature, these rings, sectors of ring or studs are situated above the above-mentioned trench(es) separating the shield(s) called the first shield(s) from the associated contact(s).